I have a 54 gallon corner aquarium, that came with a little All-Glass Aquarium twin tube light (24″ tubes, 15 watts each), which isn’t much light for a fairly large/deep tank. A friend of mine who is very fishy-equipment-smart suggested getting a retrofit kit for it, which is what I ended up doing. Aquarium Hobbyist Supply sells a kit that for a 55 Watt compact fluorescent bulb that looked like it would fit into my existing plastic enclosure, so I ordered it, procrastinated for 3 and a half months, and just assembled it.
It is not a drop-in replacement for the existing twin tube light, there were bits of plastic that supported the old bulb assembly inside the casing that I had to cut away with a pair of wire cutters. Once these were trimmed down a little bit, the new polished aluminum reflector fit in without a problem, and I could reuse two of the existing screw holes to attach it. There was still just enough space in the enclosure to also fit the electronic ballast. Wiring everything up is very easy, just follow the color-coded wiring diagram. Everything you need is included in the kit (wire nuts, cable clamps, ground screw attachy thingy, etc). The final assembled light actually looks like something you could buy, not something put together by hacking apart a basic plastic light (pictured with the old light fixture):
I have a 6700k bulb in the light at this point, it is very bright (compared to the old light, by aquarium standards this is still a not very powerful light), and the plants appear a much brighter shade of green. It does not appear to be getting alarmingly hot, likely helped by the existnig light already having vent holes. The only drawback of the light is that it really lights up any debris in the tank, no skipping water cleanings with this thing.